Conventional glass-melting processes entail depositing pulverulent or granulated batch material onto a pool of molten glass in a furnace and supplying heat to the batch material and molten glass contained in the furnace by the combustion of fuels using preheated air in a space above the molten glass and glass batch material so that the flame passes over them, causing a transfer of heat to them. Additional heat is typically provided to the molten glass by using electric heaters.
In conventional glass-melting furnaces, a chamber for containing the molten glass is provided. This chamber comprises the bottom of the furnace, two spaced side walls, a first end wall and a second end wall which is spaced from and opposite to the first. The chamber is confined from above by a crown exhibiting a curve across the furnace between the side walls. In the vicinity of the first end wall, raw batch materials are fed through an opening into the furnace, and completed molten glass is removed from the furnace in the vicinity of the second end wall for further forming.
Fuel-fired furnaces fall into two types: side-fired and end-fired. The side-fired furnaces are provided on their outside with a heat recovery system including regenerators or recuperators. In the side-fired furnaces, firing direction does not influence glass quality; they are less sensitive to charging techniques; but they have higher construction cost, lower fuel efficiency and require more room. As to the conventional end-fired furnaces, they have better fuel efficiency, lower construction cost, require less room, but generally have lower glass quality because the firing direction influences charging patterns.
As discussed in U.S. Pat. No. 4,559,071--Kunkle et al, melting glass in tank-type furnaces has a number of drawbacks. A main drawback is the need to carry out several partly incompatible operations simultaneously within the same chamber. Thus, the melter chamber is expected to liquefy the glass batch material, to dissolve grains of the batch, to homogenize the melt product, and to refine it by freeing it of gaseous inclusions. As a result, inhomogeneities exist within the melt product because of different melting temperatures of the different components of the glass. In order to decrease such inhomogeneities, the usual tank-type furnace contains a relatively large volume of molten glass in which recirculating flows are provided. Maintenance of necessary temperatures both for said recirculating flows and for a large chamber results in inefficient use of thermal energy. Furthermore, it is known that some components of the batch such as limestone tend to melt out earlier than the sand and sink into the melt as globules, whereas sand tends to form a residual unmelted scum on the surface of the melt. This increases inhomogeneities in the melt.
A major rate-limiting step of the melting process is the rate at which partly melted liquefied batch runs off the batch pile to expose underlying parts of the batch to the heat of the furnace. The conventional process of floating a layer of batch on a pool of molten glass is not particularly conducive to aiding the runoff rate, in part because the batch is partially immersed in the molten glass. Convective heat from the pool of molten glass induces considerably less runoff than radiant energy. Conventional overhead radiant heating is inefficient because only one side of the batch is exposed to overhead radiant heat sources, and only a downwardly directed part of radiant energy heats the batch. Considerable energy is wasted through the superstructure of the furnace and causes thermal degradation of the refractory roof components.
Furthermore, in all furnaces that I am aware of, it is not feasible to produce more than one type of melt in one furnace at the same time.
Some proposals have been made to overcome some of the problems of the conventional tank-type melting furnace by way of isolating the initial process of liquefying batch material from the remainder of the melting process (e.g., U.S. Pat. Nos. 4,381,934; 4,492,594; 4,496,387; 4,539,034; 4,559,071; 4,604,121; 4,634,461; 4,654,068). In U.S. Pat. No. 4,559,071, for example, there is proposed a combined plant containing at least one special vessel or drum having a burner (for carrying out the process of liquefying batch material) and disposed below it a tank-type furnace (for providing the next melting operations). Said tank-type furnace is supplied with the additional means for heating. According to this U.S. Pat. No. 4,559,071, this method and furnace provide better fuel efficiency, i.e., 5.4 million BTUs per ton of glass produced versus 6.25 million BTUs per ton of glass produced in the conventional glass-making tank-type furnace. A disadvantage of the above type of combined plant is that it has a small throughput. To provide liquefied batch to a furnace of large scale, using a plurality of smaller liquefying units is considered more economical than using a large single liquefaction vessel. But the size of the liquefied batch charge zone increases as the number of liquefying units charging the furnace grows. To provide liquefied batch to a 500-ton capacity furnace by utilizing a plurality of the largest of said liquefying units (U.S. Pat. No. 4,604,121) with 30 tons per day throughput and a 130 cm inner diameter, I calculate that a charge zone of at least 60 square meters is needed. So, usage of the above type combined device for large capacity furnaces does not seem feasible.
Another proposal for heat pretreating glass batch for the liquefaction process is made in U.S. Pat. No. 4,604,121. In this patent, a rotary drum for feeding batch material is connected to a special vessel positioned below it, where batch material is partially melted as a first step in the melting process. To complete the melting process, a tank-type furnace, fed from the special vessel, is used. Exhaust gases from the special vessel are used for preheating the batch material in the drum. This combined plant has actually three type devices, which leads to higher maintenance and construction costs.
In USSR patent specification No. 842,059, issued June 30, 1981, a tank-type end-fired furnace for melting of rock batch, such as basalt rock, is proposed. This tank-type furnace comprises side and end walls, a bottom and a crown supported by said end walls and exhibiting a curve along the furnace, while the charge ports are formed between the side walls and the sides of the roof in the furnace cross section. Two arch bridges between each side wall and the bottom form two channels connected near one end wall with the inner gas space that is located within the confines of the furnace walls.
Batch feeding is provided through charge ports along the furnace. While gradually moving down along the side walls, the rock batch is exposed to the radiant energy of the burner, and is thus converted into melt that flows down to the bottom, then flows along the furnace to the exit end wall. Such a furnace design allows an increase in the specific output, or specific melting capacity and provides heat recovery from exhaust gases by preheating burner supply air and by additional batch heating through the walls of the said channels. However, this furnace is limited in its application to the processing of one-component rock batch, which requires only a one-step operation for conversion into a completely molten product. To produce melt from multi-component batch such as batch for different sorts of glass or fiber glass, more operations, including refining the melt by freeing it of gaseous inclusions, are required.